I have a 2000 Grand Marquis and when I have the AC or Heat on the fan does one of three things. Runs full blast, doesn't run at all, or runs normally and at that time the speed can be controlled with the speed control dial. At times when the fan is running full blast, it can turn off and on multiple times in a short period of time and sometime comes on for about a second. Any ideas what is causing this and how to fix it?